The subject matter disclosed herein generally relates to cooling and purge air flow systems for gas turbines and more particularly to the reduction of performance loss from cooling and purge air flow systems for gas turbines.
A conventional gas turbine system, includes a compressor section, a combustor section, and a turbine section. In a conventional gas turbine system, compressed air is provided from the compressor section to the combustor section. The air entering the combustor section is mixed with fuel and combusted. Hot gases of combustion flow from the combustor section to the turbine section to drive the gas turbine system and generate power. The turbine includes one or more stages of stator nozzles or vanes, rotor blades and annular shrouds around the turbine blades to maintain appropriate clearances.
A first turbine stage nozzle receives hot combustion gas from the combustor. The hot combustion gas is directed to the first turbine stage rotor blades to extract energy. A second turbine stage turbine nozzle may be disposed downstream from the first turbine stage rotor blades, and is followed by a row of second turbine stage rotor blades that extract additional energy from the combustion gas. Additional stages of turbine nozzles and turbine rotor blades may be disposed downstream from the second stage turbine rotor blades.
A cooling fluid such as air is provided to the turbine vanes, blades, and shrouds to maintain the temperatures of those components at appropriate levels to ensure a satisfactory useful life of the components. Cooling is typically accomplished by extracting a portion of the compressed air from the compressor and conducting it to the components of the turbine. The rotating blades include hollow airfoils that are supplied the cooling air through cooling circuits. The airfoils include a cooling cavity bounded by sidewalls that define the cooling cavity. Any air compressed in the compressor and not used in generating combustion gases will reduce the efficiency of the engine. Therefore, it is desirable to reduce the amount of cooling air bled from the compressor. Furthermore, the air used for cooling turbine components typically discharges from orifices or gaps in those components. That cooling air mixes with the combustion gases in the turbine and will also reduce engine efficiency for thermodynamic and aerodynamic reasons. Accordingly, while turbine efficiency increases as turbine inlet temperature increases, that increase in temperature also requires effective cooling of the heated components, and such cooling is effected in a manner so as not to forfeit the increased efficiency realized by the increased temperature.
The axial location or stage where the air is bled from the compressor is determined by the pressure required by the component or system to be serviced by that air. To ensure sufficiently high delivery pressure, in general, it is desirable to select the source with the highest possible pressure. However, bleeding air from the earliest possible stage of the compressor will increase compressor efficiency by reducing the amount of work invested in the extracted air. Therefore, it is desirable to achieve the highest possible system supply pressure from the earliest and the lowest pressure stage of the compressor.
Modern systems utilize variable extraction through modulation valves to provide the amount of flow required. The flow is determined based on needs for cooling, backflow margin (BFM), wheelspace temperature, and maximizing turndown.
Furthermore, the cooling air must be provided at suitable pressures and flow rates to not only adequately cool the turbine component(s), but to maintain an acceptable BFM. BFM is defined as the difference between the pressure of the coolant inside the airfoil and the local pressure of the combustion gases outside the airfoil. Sufficient BFM must be maintained to prevent ingestion of the hot combustion gases into the airfoil, and ensure continuous discharge of the coolant through the airfoils. An adequate BFM limits leakage of hot gases from the gas path. Loss of gases flowing along the gas path leads to a reduction in output from the gas turbine system and may cause damage to secondary flow/cooling components resulting from hot gas ingestion.
During operation of a gas turbine, the BFM requirement typically determines the required amount of air extracted from the compressor. Currently, the BFM is set based on a probabilistic study and is applied to a unit controller as a pressure ratio demand which is set as a constant value over the life of the turbine. This approach fails to take into account the variation in the BFM over the life cycle of the component. Consequently, the air pressure within the outer side wall cavity associated with the component may be higher than necessary, resulting in less than efficient operation.